The present invention represents a further development and improvement of a prior machine tool for machining a crankshaft as disclosed in the U.S. Pat. No. 3,795,161 granted on Mar. 5, 1974 jointly to the inventor of the instant case and to another inventor. The disclosure of such prior patent is to be considered part of the disclosure of the present invention. The present invention and the prior invention both relate to a machine tool for machining a plurality of co-axial cylindrical shaft surfaces, such as the journal surfaces of a crankshaft. For this purpose an annular milling tool is used which is provided with internal cutting edges and orbits during a machining operation about the axis of the shaft which constitutes the workpiece.
The present invention further relates to the annular rotary tool of the type having internal cutting edges for simultaneously machining a plurality of co-axial surfaces of the work.
The prior machine has two longitudinal slides which are displaceable on a guideway on a bed of the machine tool, the bed extending parallel to the axis of the workpiece to be machined. Each slide carries a tool unit comprising a cross-slide carrying the annular tool and driving means which rotate the tool about its own axis and at the same time cause the tool to orbit about the axis of the workpiece. In the case of the prior machine tool, each annular milling tool employed therein has only one circular set of internal cutting edges and can therefore machine only one cylindrical shaft surface at a time, a second surface being machined simultaneously by the tool unit mounted on the other longitudinal slide. Hence, it is not possible to machine more than two spaced line bearing shaft surfaces simultaneously with the prior annular tool on the prior machine tool. There is not room enough in that machine tool for placing the tool units so close enough to each other as would be generally required for the simultaneously machining of more than two co-axial line bearing shaft surfaces.
It is an object of the present invention to provide an annular milling tool of the kind described above capable of simultaneously machining at least four co-axially spaced cylindrical shaft surfaces, in particular the surfaces of the journals for the line bearings of a crankshaft.
It is a further object of the invention to provide a machine tool with an annular milling tool having internal cutting edges capable of simultaneously machining of at least two co-axially spaced cylindrical shaft surfaces, in particular the surfaces of the journals of the line bearings of a crankshaft.
In accordance with the present invention an annular milling tool for simultaneously machining a plurality of co-axially spaced shaft surfaces by the orbiting method has at least two circular sets of internal cutting edges with gaps for chip removal between said sets. It is possible to employ the annular milling tool according to the invention on the prior machine tool as described in U.S. Pat. No. 3,795,161. In this case all of the co-axially spaced cylindrical shaft surfaces of the journals on one half of a crankshaft can be simultaneously machined by the novel annular milling tool carried by the tool unit provided on one of the two longitudinal slides of the aforesaid machine tool and all the co-axially spaced cylindrical shaft surfaces of the journals on the other half of that crankshaft can be simultaneously machined by the novel annular milling tool carried by the tool unit provided on the other one of the two longitudinal slides.
The accuracy of machining may be increased by incorporating certain features into a machine tool which is operable with an annular milling tool according to the invention by the orbiting method. In accordance with a further feature of the invention such a machine tool is organized as follows. Means are provided for rigidly supporting the work shaft approximately at its centre in stationary condition. If the shaft is a crankshaft, it is preferred to provide means for causing the two annular milling tools to rotate about their respective axes in opposite directions and to arrange said axes so that they are spaced apart by 180.degree. away from each other on their respective orbits during their rotation about the axis of the crankshaft. The bending moments exerted on the work by one of the two annular milling tools will then act on the workpiece in a direction opposite to that of the bending moments exerted by the other annular milling tool so that the bending and torsional stresses exerted on the workpiece during the milling operation are reduced.
Machining accuracy may also be increased by providing in a machine tool according to the invention means which make it possible for a workpiece to be machined in two stages, a rough-cutting stage and a finishing stage. It is desirable that these two stages should be performed on the same machine tool without making it necessary for the workpiece to be unclamped from its holder or holders. To this end, the eccenter body carrying the annular milling tool may be composed of an inner eccentric annular element and of an outer eccentric annular element, with means for angularly displacing both eccentric elements relative to one another. Such a composite eccenter body is known from the German Offenlegungsschrift No. 1,552,803 in connection with the machining of crankshafts. In this known arrangement, the outer eccentric element can be adjusted to move the annular milling tool to a position in which that tool is co-axial with the main axis of the crankshaft and can be passed lengthwise along the crankshaft. In the machine tool described in the prior patent above referred to, the cross-slide is used for the purpose of bringing the annular tool to such co-axial position. In order to make it possible for this known arrangements to be used for rough-cutting or for finishing, as desired, a motor driven shaft is connected by a first train of motion transmitting elements to one of the eccentric annular elements of the eccentric body and a second train of motion transmitting elements is connected to the other one, preferably with the inner one, of the two annular eccentric elements. A clutch is provided which may be so shifted as to optionally connect the second train of motion transmitting elements either with the motor driven drive shaft or with a stationary braking element. In the latter alternative the second train of motion transmitting elements and the inner annular eccentric elements are arrested while the outer annular eccentric element is turned by the drive shaft so as to change the eccentricity of the composite eccentric body. This eccentricity, i.e. the distance of the axis of rotation of the eccentric body from the axis of rotation of the annular tool, determines the diameter of the workpiece.
By virtue of this selective change-over, it is possible when rough-cutting a cylindrical surface to leave enough material below that surface to allow it to be finished without great cutting forces being required, so that the elastic deformations of the workpiece and tool will be negligible.
If the machine tool according to the invention is so designed that the clutch can be engaged and disengaged during machining, then the change-over from rough-cutting to finishing can take place without interrupting the cutting operation.